1. Field of the Invention
The present invention relates to an etchant for use in association with metal silicides and, more particularly, to an etchant which comprises a mixture of ethylene glycol and hydrofluoric (HF) acid and is capable of controlling the etch rate of metal silicides, and in particular titanium silicide, such that titanium silicide may be used in place of tantalum silicide in the formation of integrated circuits without requiring any modifications of the conventional etching process associated with the use of tantalum silicide.
2. Description of the Prior Art
In the past, polycrystalline silicon, referred to as polysilicon, was utilized in the manufacture of integrated semiconductor devices to form the gates for active devices and the interconnections between the devices. However, the RC time constant associated with long runs of polysilicon interconnect was often the dominant delay-producing element. Therefore, as the demand for greater packing density and complexity increased, the signal propagation delay associated with the sheet resistance of polysilicon (which is on the order of 30 to 60 ohm/square) led to the search for an alternative material which could be used in place of or in conjunction with polysilicon. Refractory metal silicides such as TiSi.sub.2, WSi.sub.2, MoSi.sub.2, and TaSi.sub.2 are considered viable alternatives to polysilicon. U.S. Pat. No. 4,276,557 issued to H. J. Levinstein et al on June 30, 1981, discloses an integrated structure and method of manufacture which utilizes either TiSi.sub.2 or TaSi.sub.2 as a substitute for polysilicon. However, Levinstein et al does not discuss any of the problems associated with the volatility of TiSi.sub.2 in the presence of the conventional etch solution consisting of buffered hydrofluoric (HF) acid. The reactivity of titanium silicide with buffered HF is discussed in the article "Refractory Silicides of Titanium and Tantalum for Low-Resistivity Gates and Interconnects", by S. P. Murarka et al appearing in IEEE Transactions on Electron Devices, Vol. ED-27, No. 8, August 1980, at pp. 1409-1417. In discussing the etching of tantalum and titanium silicide, Murarka et al states that the approximate overall etch rate of TaSi.sub.2 and TiSi.sub.2 films in a 10:1 BHF (buffered hydrofluoric acid) solution are 300 .ANG./minute and over 1500 .ANG./minute, respectively. In association with this excessive etch rate of titanium silicide, any conventional amount of this material which would be used for gates and interconnects would thus be completely removed after being immersed in 10:1 BHF for about one minute. Therefore, any process which uses titanium silicide as a metallization layer needs to be monitored closely since any slight operator miscalculation will result in the complete removal of the titanium silicide material. In light of this problem, Murarka et al recommends that the use of titanium silicide should be limited to processes where the metallization would not be exposed to any HF-containing solutions.
However, it is well known that titanium silicide has a lower resistivity than tantalum silicide (approximately 15-25 .mu.-cm versus 60-75 .mu.-cm). Therefore, it is desirable to fine some method of using titanium silicide in place of tantalum silicide which does not require extreme processing controls during the integrated circuit manufacturing process.